The present invention relates to a scintillation camera, and particularly to a scintillation camera comprising a circuit for correcting a positional signal and energy signal which are obtained by detecting .gamma. (gamma) rays emitted from a subject.
A scintillation camera is suitably used as imaging means for a nuclear medical diagnostic apparatus such as a single photon ECT (SPECT).
Such a scintillation camera obtains two-dimensional positional information and energy information of .gamma.-rays emitted from the nuclide administered to a subject. When the .gamma.-ray enters the scintillation camera, a scintillator absorbs the energy of the .gamma.-rays and emits fluorescent light. The fluorescent light simultaneously enters a plurality of photomultiplier tubes (PMT) through a light guide. The incident light is photoelectrically converted into electrical pulses which are output to a position/energy computing circuit from the plurality of PMT. The position/energy computing circuit computes the position and energy of the incident .gamma.-rays on the basis of the plurality of electrical pulse signals, and outputs digital positional signals x, y and energy signal z to a correction circuit in the subsequent step.
The correction circuit comprises an energy signal correcting circuit and a linearity correcting circuit for the positional signals. The energy signal correcting circuit corrects a distribution curve position of the energy spectrum of the energy signal which changes with the incident position of the .gamma.-rays. The linearity correcting circuit performs vector operation of the positional signals x and y by using a correction vector for each of two-dimensional positions X and Y.
At least one set of correction vector data is previously prepared for correcting the linearity. The correction vector data are previously set in one-to-one correspondence with the energy spectrum distribution of one or more nuclides used. An energy window (referred to as a "specified energy window" hereinafter) of set % (for example, 20%)is assigned to each of the energy spectrum distributions.
If four specified energy windows W.sub.0 . . . W.sub.3 are provided for several nuclides, as shown in FIG. 1 (wherein the horizontal axis represents energy values of .gamma.-rays and the vertical axis frequencies of each energy value), four sets of correction vector data L.sub.0 . . . L.sub.3 are previously prepared. One of the four sets of correction vector data L.sub.0 . . . L.sub.3 is selected by discriminating the magnitude of the raw energy signal z supplied from the position/energy computing circuit. Namely, one of the specified energy windows W.sub.0 . . . W.sub.3 which corresponds to the magnitude of the energy signal z is decided, and the correction vector data L.sub.0 (. . . L.sub.3) corresponding to the decided specified energy window W.sub.0 (. . . W.sub.3) is selected for each incidence of .gamma.-rays. Real-time correction of linearity is performed for the positional signals x and y by using the selected set of correction vector data.
However, the conventional correction of linearity has the problem that the correction is roughly made because only a set of correction vector data is used for a spectrum distribution.
In recent years, diagnosis has frequently been made by using unexpected nuclides or simultaneously administering two types of nuclides due to diversification of the approach to nuclear medical diagnosis.
In such a case, the center of a spectrum distribution curve deviates from the center of a single energy window (curve DB.sub.2), or two spectrum distribution curves simultaneously appear (curves DB.sub.1 and DB.sub.2), as shown in FIG. 1.
For instance, since the curve DB2 spreads over two specified energy windows W.sub.0 and W.sub.1, the correction vector data L.sub.0 or L.sub.1 is selected according to event, and the selection is thus unstable. When two spectrum distribution curves DB.sub.2 and DB.sub.3 are present because two nuclides are used, the same problem occurs.
Further, the linearity of the positional signals of .gamma.-rays having an energy value at the central portion of one specified energy window W.sub.0 (. . . W.sub.3) is relatively well-corrected because the correction vector data are set to the central position. However, correction accuracy deteriorates due to outward deviation from the central portion in the same specified energy window.
The variation and deterioration in the correction accuracy are directly connected with deterioration in uniformity and resolution of the photographic image formed. Namely, periodicity remains in the resolution, and the resolution itself deteriorates, thereby making it impossible to comply with recent demands for increasing precision and quality of the image formed.
When a photomultiplier tube having a large bore (for example, an incident surface having a diameter of 3 in.) is used, this problem becomes particularly significant, thereby causing difficulties in achieving compatibility between resolution and uniformity.
On the other hand, in the above-described conventional technique, since one of the correction vector data is selected on the basis of the raw energy signal z (i.e., uncorrected energy signal) output from the position/energy computing circuit, the selection of correction vector data often produces error. This also deteriorates the uniformity of resolution and the resolution itself.
The present invention has been achieved for solving the above problems of conventional technique, and an object of the invention is to improve the periodicity (uniformity) of positional resolution and the positional resolution itself by high-precision correction of linearity of positional signals in accordance with the energy value of .gamma.-rays wherever a spectrum distribution curve of .gamma.-rays is present.
Another object of the present invention is to improve the precision (uniformity, resolution) of linearity correction by selecting data for linearity correction on the basis of an energy signal closer to a true value.
A further object of the present invention is to achieve compatibility between uniformity and resolution of an image even when a photomultiplier tube having a detection surface having a relatively large diameter is used.